Pressurized-water and boiling-water nuclear reactors utilize a vessel to contain the fuel rods with their associated fissionable material. Coolant water is circulated through the vessel and is heated (PWR) or partially vaporized (BWR) by heat transfer from the fuel rods. The heated coolant of a PWR is utilized to vaporize a secondary through heat exchangers. In the power generation application of nuclear reactors, the vapor powers a steam turbine rotating a generator.
In the event that a malfunction of the system takes place so that the normal coolant circulation is interrupted, the reactor is shutdown by inserting absorber rods into the core, thereby interrupting the nuclear reaction. However, even after shutdown of the reactor, the fission products in the fuel rods continue to produce heat generally referred to as "decay heat". Without the normal coolant flow, this decay heat could melt the fuel rod cladding, the fuel, and the vessel itself, releasing radioactive fission products into the secondary containment building and thereby increasing the risk that radioactive material would be introduced into the atmosphere with the associated potential danger to the public.
The potential for release of radioactive material into the environment has led to the development of emergency core-cooling systems. All nuclear reactors must now have provision for maintaining sufficiently low temperatures after a malfunction that the integrity of the fuel rods will be insured. The primary malfunction with which the emergency core-cooling systems are concerned is a loss-of-coolant accident. In such an accident, the primary coolant system develops a leak or rupture resulting in some of the primary coolant water being lost from the system. When the leak is a relatively minor one, the primary coolant system can continue to function to cool the core after the shutdown so long as the small quantity of coolant being lost is replenished. The replenishment of coolant through a small leak is accomplished by a high-pressure injection system. However, in the event of a large rupture developing in the primary coolant system, a different emergency core-cooling system becomes effective. According to conventional design, such an emergency core-cooling system operates in two distinct phases. Initially, accumulator tanks are discharged and/or pumps operated to rapidly refill the vessel with borated coolant water. Subsequently, the new coolant is circulated through the pressure vessel. Steam leaking into the reactor containment is condensed, removing the reactor heat from the system. Power for the pumps is obtained from an independent power source such as a Diesel engine. Typically, a complete emergency core-cooling system injects water into each of the primary coolant loops of the reactor so that the break in a single coolant loop will not defeat the operation of the emergency core-cooling system, pumping water into the other primary coolant loops.
It will be apparent that the provision of such systems sufficient to maintain a safe temperature within the vessel is an expensive and critical component of the overall power generating system. Such conventional emergency core-cooling systems are rendered more expensive and less reliable because each of the possible contingencies for such a system's operation adds further to the design capacity requirements. For example, Diesel generator sets have a high startup failure rate, requiring a backup electric system to increase the reliability. A critical time lag may develop before the cooling water from the pumps is injected. Water coming into contact with the hot core is then partially vaporized by the fuel elements. The steam in the vessel collects in the plenum of the vessel producing a back pressure to the entry of new coolant water. The steam problem, generally referred to as "steam binding", limits the design of the reactor plant so that the initial temperature rise caused by the steam binding effect does not endanger the integrity of the reactor core.
Therefore, it is desirable to have a system for removing heat from the vessel that increases the reliability of decay heat rejection from the reactor after an accident and has a short startup time. Such a system is particularly desirable if it is capable of operating independently of an electrical power source.